A 300 KB RAR archive labelled X73-MB-D, which most likely refers to a motherboard identifier with revision D, though the name gives no brand or file type. Treat its contents as unconfirmed until opened and inspected. Files of this size often contain a boardview, a small BIOS image or an EC firmware file. Check the extracted extension, open boardviews in a viewer such as OpenBoardView, and for any firmware make sure the board marking matches and that the original chip is read and saved before writing new data.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @freeschematicdiagram. Enginerys does not host this file.
How to open a .rar file
Compressed archive — the usual wrapper for a schematic + boardview pair or a BIOS + EC set.